The mystery of human memory

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[Added September 2006]
The last scientific result about memory and sleep :A daytime nap containing solely non-REM sleep enhances declarative but not procedural memory leads to the following reasoning. Why humans (and
animals) sleep? The obvious answer : to help our body recover is not an answer.
In fact it is enough to stay quiet without sleeping in order to recover from
stress. The evidence is mounting that the only purpose of sleep is to enhance
our memory. This explains also why childes sleep more (they have to learn a lot).

[Added January 2005]
Normal sleep consists typically of periods lasting around 90 minutes that repeat 4 or 5 times each night. Each period starts with a non-REM phase where we
go through 4 stages of sleep until we reach the stage of deep sleep with low
frequency electrical brain waves. Then the REM sleep starts with a lot of brain
activity and rapid eye movements. This ends when we get awake for usually a very short time and then a new cycle of sleep starts: light sleep, deep sleep, REM sleep.

Early experiments have shown that rem sleep doesn’t have any influence
on remembering lists of words or facts(i.e. declarative memory).
Instead rem sleep influences procedural memory (like the ability
to recognize patterns in a computer screen).Motor skills which depend
on procedural memory, are affected also by non-rem deep sleep.

Skills that require visual abilities are improved by both rem and
non-rem deep sleep.Sometimes also an hour with shut-eyes makes a big
difference.It seems that different kinds of memory need different kinds
of sleep.This may explain why sleeping on a problem can produce sometimes
the result of getting awake the next morning with a solution.
Studies in animals have shown that during the sleep the same neurons
activated during the day by doing some task like traversing a maze, are activated again.
It seems as if during the night we are re-enacting (simulating) the same
experiences that we had during the day. This seems to produce two results:

reinforce some pathways making permanent some memories

drop memories connected to experiences considered irrelevant

This second task may be the result of the work done during slow waves deep
sleep when connections between neurons seem to be indiscriminately weakened.
So the sleep is perhaps a series of repeated cycles of pruning and strengthening neural connections that let you learn new skills without forgetting old ones.

Since we dream so much, why the dreams we remember are so few? The answer is
astonishing:it is true that we dream a lot of time but most of this time
the brain circuitry needed to register those dreams is off. Only in the very
few moments that we get awake during the night, we are able to register
a few dream images , That’s all.It seems that every night, also if we think
that we slept all night, there are a few moments where ,for few seconds, we get
awake and can register the end of the last dream. This explains why,with some
training, everyone can remember many dreams every night.

[Added November 1997]
According to recent studies,dreams may reflect a fundamental aspect of mammalian
memory processing(only mammals dream).
The long term memory of crucial information acquired during the waking
state seems to require the reprocessing of this information during the
REM phase of sleep when we dream.

To avoid interference with sleep of the re-enacted experience the motory
neurons that command our muscles are inhibited, except for those that
command our eye movement. Everything seems to start with activation
of hippocampus a part of the brain involved with memory processing.

The kind of memory which is more influenced by sleep and dreams is
the implicit procedural memory. This should be “obvious” from the fact
that also animals dream and animals have only this kind of memory.